Devarim - Sh'vi'i
“There’s No Need to Fear!”
Moshe told the Tribes who inherited the land in Trans-Jordan that they were receiving their portion conditionally. They had to cross into Israel and fight the Canaanite nations alongside the other Tribes. They could leave their families and flocks in their cities, but they couldn't join them there until all the Tribes had also been given their portions.
Moshe also told Yehoshua to take note of what God had done to Sichon and Og. That's exactly what He would do to the Canaanite kings, so Yehoshua had no reason to fear.